Traditionally, cellulose fibers from southern pine and other softwood species are used in absorbent products in large part because the morphology of these fibers provides good absorbent performance. Compared to hardwood fibers, southern pine and other softwood fibers tend to be longer (e.g., having a length weighted fiber length of about 2.5 mm) and more coarse (e.g., having a coarseness greater than about 20 mg/100 m), and form low density pads with sufficient void volume to hold several times their weight in liquid. Hardwood fibers, on the other hand, are known for their performance in paper applications where shorter fiber length (e.g., about 1 mm) and lower coarseness (e.g., less than about 20 mg/100 m) provide a dense structure and smooth paper surface.
Crosslinked cellulose fibers are usually produced by applying a crosslinking agent to a dried sheet or roll of conventional softwood pulp fibers, generally at a dilute concentration to ensure chemical impregnation of the sheet, followed by wet fiberization in a hammermill to generate treated, individualized cellulose fibers. These fibers are then dried, such as in a flash drier, and cured, such as in an oven. The resulting fibers exhibit intrafiber crosslinking in which the cellulose molecules within a cellulose fiber are crosslinked. Intrafiber crosslinking generally imparts twist and curl to the cellulose fiber, and also imparts bulk to the fiber, properties that are advantageous in some absorbent products.
One drawback of this method is the high capital cost of the production process, as well as high energy costs due to drying the fiber prior to curing. Another drawback is that wet hammermilling can generate fiber and chemical buildup under usual mill conditions of heat and high airflow. Additionally, wet hammermilling produces undesirable features such as knots, which are unfiberized fiber clumps or pieces of the original pulp sheet. Generally, as production speeds increase, the level of knots also increases as the hammermilling efficiency is reduced.